1. Field of the Invention
The present invention relates to a substrate cleaning apparatus cleaning a semiconductor substrate, a glass substrate for a liquid crystal display, a glass substrate for a photomask or a substrate for an optical disk (hereinafter simply referred to as “substrate”) held on a rotary base while rotating the same in a substantially horizontal plane.
2. Description of the Background Art
In general, the surface of the substrate must be kept extremely clean in a manufacturing process for the aforementioned substrate, and hence cleaning processing such as that or removing particles adhering to the surface of the substrate is performed in each of various steps in the manufacturing process.
A cleaning system discharging a cleaning mist prepared by mixing a detergent (or cleaning liquid) and pressurized gas with each other to the substrate through a double-fluid nozzle is known in relation to such cleaning processing.
This cleaning system not directly coming into contact with the substrate is capable of cleaning the substrate without physically damaging a pattern or the like. The discharge rate for the cleaning mist can be controlled with a relatively large variable width (from 30 m/s to about the sound velocity) by adjusting the flow rate of the gas, and hence cleaning can be performed in response to the object substrate by controlling the discharge rate.
When the discharge rate for the cleaning mist is increased to about the sound velocity, for example, a sufficient cleaning effect can be attained simply by using only deionized water as the detergent without using acid or alkali. When the discharge rate is relatively reduced to the contrary, a damage applied to the object substrate can be relatively reduced.
It has been proved that watermark defects are caused to increase the quantity of foreign matter when the aforementioned cleaning system is applied to an object substrate having a hydrophobic surface, for example. The number of such defects is increased toward the peripheral edge portion of the substrate subjected to relatively large centrifugal force resulting from rotation.
The inventor has made deep studies on the cause for the watermark defects, to find out that the cleaning mist reaching the substrate is splashed to adhere to the substrate again and dried as such to result in the watermark defects in the cleaning system discharging the cleaning mist prepared by mixing the detergent and the pressurized gas with each other to the substrate. It has also been proved that a small flow rate of the detergent also results in watermark defects in this cleaning system.